In a conventional bipolar device including a semiconductor layer formed of silicon carbide (SiC), a temperature coefficient of resistance provided when a current flows through a pn junction in a forward direction is negative. In a case where many devices are connected in parallel and are caused to operate, variation in voltage drop among the devices causes a current to concentrate on a device in which relatively great voltage drop occurs. Then, as for devices each having a negative temperature coefficient of resistance at a forward-current-carrying time, in a device on which a current concentrates, resistance is reduced due to temperature rise, so that a current further concentrates thereon. As a consequence, the device is broken down in some cases.
On the other hand, also with regard to a device having a positive temperature coefficient of resistance, in a case where many devices are connected in parallel and are caused to operate, variation in voltage drop among the devices causes a current to concentrate on a device in which relatively great voltage drop occurs. However, when resistance is increased due to temperature rise in the device on which a current concentrates, current concentration is alleviated. As a consequence, a stable operation can be expected in a case where many devices are connected in parallel and are caused to operate.
In view of the above-described matters, as a technique for preventing a current from concentrating on a part of devices in a case where many bipolar devices are connected in parallel and are caused to operate, a configuration in which a resistor having a positive temperature coefficient of resistance is connected in series with a bipolar device is disclosed (Patent Document 1, for example). According to Patent Document 1, an n-type substrate layer which is doped at a low concentration is placed as an epitaxial layer of a device. As a result of providing such a configuration, resistance of the substrate layer contributes to voltage drop of the device as a whole, so that a temperature coefficient of resistance provided when a current flows in a forward direction is positive.